Investigation of Microstructure and Mechanical Properties of Multi-Pass Friction Stir Processing on Precipitation Hardened Aluminum Alloy

Friction stir processing is a prominent technique for microstrucutral refinement of metallic materials. In the present study, multi-pass friction stir processing of aluminum AA2014-T651 alloy is performed with one and four passes. The multi-pass processing is performed at a tool rotational speed of 1000 rpm with a traverse speed of 63 mm/min. Microstructural characterization of friction stir processed specimens indicated a significant grain refinement with equiaxed grains in the four-pass friction stir processed specimen. Nonetheless, mechanical properties exhibited a decreasing trend on increasing the number of passes. The microhardness, tensile strength and fracture toughness of the multi-pass specimen decreases with the increase in number of passes. Hardening precipitates in the base alloy govern the mechanical properties and contribution of the grain refinement to strength, hardness and fracture toughness is limited. Thermal cycles experienced in the multi-pass friction stir processing affect the role of hardening precipitates.